Change (apply 'nconc (mapcar ...)) to (mapcan ...); warn about first form. lisp/ChangeLog addition: 2010-09-16 Aidan Kehoe <kehoea@parhasard.net> * byte-optimize.el (byte-optimize-apply): Transform (apply 'nconc (mapcar ...)) to (mapcan ...); warn about use of the first idiom. * update-elc.el (do-autoload-commands): * packages.el (packages-find-package-library-path): * frame.el (frame-list): * extents.el (extent-descendants): * etags.el (buffer-tag-table-files): * dumped-lisp.el (preloaded-file-list): * device.el (device-list): * bytecomp-runtime.el (proclaim-inline, proclaim-notinline) Use #'mapcan, not (apply #'nconc (mapcar ...) in all these files. * bytecomp-runtime.el (eval-when-compile, eval-and-compile): In passing, mention that these macros also evaluate the body when interpreted.

--- a/lisp/frame.el	Thu Sep 16 13:36:03 2010 +0100
+++ b/lisp/frame.el	Thu Sep 16 13:51:49 2010 +0100
@@ -861,7 +861,7 @@
 (defun frame-list ()
   "Return a list of all frames on all devices/consoles."
   ;; Lists are copies, so nconc is safe here.
-  (apply 'nconc (mapcar 'device-frame-list (device-list))))
+  (mapcan #'device-frame-list (device-list)))
 
 (defun frame-type (&optional frame)
   "Return the type of the specified frame (e.g. `x' or `tty').